The present invention relates generally to an organic electroluminescent light emitting device (which will hereinafter be often called an organic EL device for short), and more particularly to an anode for supplying positive holes to a light emitting layer.
In recent years, organic EL light emitting devices have been under intensive investigation. Referring to FIG. 3, there is illustrated one typical organic EL light emitting device. Basically, the device includes a glass substrate 21 and a transparent electrode or anode 22' of tin-doped indium oxide (ITO) or the like formed on the substrate 21. A thin film 23 serving as a hole transporting layer is formed on the anode 22' by evaporating a hole transporting material such as tetraphenyldiamine (TPD). A light emitting layer 24 of a fluorescent material such as an aluminum quinolinol complex (Alq.sup.3) is deposited on the layer 23. An electrode or cathode 25 is formed thereon from a metal having a low work function such as magnesium. Such organic EL devices attract attentions because they can achieve a very high luminance ranging from 100 to 1,000 cd/m.sup.2 with a drive voltage of approximately 10 volts.
A material capable of injecting more electrons into the light emitting layer is believed to be effective for the material used for a cathode of such an organic EL device. In other words, the lower the work function of a certain material, the more suitable is the material for the cathode. Various materials having a low work function are now available. Materials usable for the cathode of EL light emitting devices, for instance, include alloys such as MgAg, MgIn, etc., as disclosed in JP-A 4-233194, and combinations of an alkali metal and a metal having a high work function such as AlCa, AlLi, etc.
When an organic EL device has such structure as shown in FIG. 3, light emission is taken out of the anode 22' side because the cathode 25 side is opaque to light. In this regard, it is noted that the cathode 25 reflects light because of being formed of a metal, etc., as mentioned above. Consequently, a part of light coming from the light emitting layer, upon reflection at the cathode 25, leaves the anode 22' side in the form of a part of emergent light, thereby making a contribution to some luminance improvement.
The light reflected at the cathode 25, on the other hand, is so scattered that it leaves the anode 22' side at an angle of emergence different from that of another part of the light coming from the light emitting layer 24. When the organic EL device is used as a display or the like, the scattered light is diverted to a non-light emitting region or a light blocking region, resulting a contrast reduction. Especially when the device is used in a brightly illuminated environment or in the sunlight, extraneous sunlight or other intense light is converted at the device into reflected light, which causes a contrast reduction and, in turn, makes it very difficult to look at the display screen.
According to one possible approach to achieving contrast improvements, an antireflection coating film is provided on a side of a substrate on which no organic EL device is formed, thereby preventing contrast reductions. However, the addition of such an extra antireflection coating film leads to increases in the number of production steps and parts and, hence, increases in the cost of the organic EL device.